The present invention relates generally to generation of X-rays, and more particularly to the multiphoton-induced production of prompt X-radiation from gas clusters. The invention was made with U.S. Government support under contracts with the University of Illinois at Chicago. The Government has certain rights in the invention.
Intense Kr.sup.9+ (4p.fwdarw.3d) emission at about 100 .ANG. has been observed from the exposure of pulsed-gas targets to high-intensity ultraviolet radiation. See, e.g., "Studies Of Multiphoton Production Of Vacuum-Ultraviolet Radiation In The Rare Gases," by A. McPherson et al., J. Opt. Soc. Am. B 4, 595 (1987). Measurements of ion production in tenuous gas targets conducted under identical conditions of irradiation, had demonstrated that no production of Kr.sup.9+ was possible from free Kr atoms at this radiation intensity. See, e.g., "Tunneling Ionization In The Multiphoton Regime," by G. Gibson et al., Phys. Rev. A 41, 5049 (1990). An identical anomaly has also been observed in studies of the 165 .ANG. emission from Ar.sup.9+. The identification of these emissions, produced either by a prompt or a delayed mechanism, were anomalous.
Estimates concerning limiting cross sections for multiphoton coupling to free atoms have been described in "Limiting Cross Sections For Multiphoton Coupling," by K. Boyer et al., Revue Phys. Appl. 22, 1793 (1987). Therein, effective cross sections for energy transfer in the high intensity limit (.gtoreq.10.sup.19 W/cm.sup.2) for heavy metals were evaluated. A prediction that energy transfer in the range between 0.1-1.0 W/atom was feasible with an intensity of irradiation between 10.sup.19 and 10.sup.20 W/cm.sup.2 emerged from this study. Such enormous values for the rate of energy transfer are comparable to those developed during subpicosecond irradiation of solid surfaces at high intensities, although it would be difficult to imagine conditions where such would occur in free atoms. In fact, X-ray emission from free rare gas atoms (e.g., Ar, Kr, and Xe) has been experimentally found to be negligible for intensities .ltoreq.10.sup.17 W/cm.sup.2 in "Search For Multiphoton-Induced Inner-Shell Excitation," by P.H.Y. Lee et al., Phys. Rev. A 40, 1363 (1989), while, by contrast, in "High Intensity Generation of 9-13 A X-Rays From BaF.sub.2 Targets," by A. Zigler et al., Appl. Phys. Lett. 59, 777 (1991), the authors observed copious amounts of kilovolt radiation from M-shell transitions of Ba ions in the dense, highly-excited plasma formed on the surface of solid BaF.sub.2 irradiated at a comparable intensity (about 10.sup.17 W/cm.sup.2).
In "Studies Of Multiphoton Production Of Vacuum-Ultraviolet Radiation In The Rare Gases," by A. McPherson et al., J. Opt. Soc. Am. B 4, 595 (1987), the authors discuss the reasons, later to be proven incorrect, that fluorescence from laser-excited rare gases is unaffected by clustering of these target gases emerging from a pulsed gas jet. Although fluorescence was observed using two different excitation pulse shapes, one of which would have been expected to produce a significant change in fluorescence distribution and quantity from clustered atoms, the patterns of fluorescence observed were essentially identical. The authors stated that experimental evidence did not lead to a conclusion that clustering of the rare gases played an important role in their observations.
In a very recent work entitled "X-Ray Generation From Nd Laser-Irradiated Gas Puff Targets," by Henryk Fiedorowicz et al., the authors describe the generation of high-intensity soft X-radiation from Nd laser-irradiated Kr or SF.sub.6 which is pulse injected into a vacuum chamber. Such X-ray sources have certain advantages over laser plasma x-ray sources using solid targets, in that target conditions are easily reproduced for high-repetition-rate applications, and there is little target debris. A Nd-glass laser was employed which generated 1-ns pulses having up to 15 J of energy. The irradiation was performed parallel to the gas flow. The authors believe that the laser radiation interacts with the aerosol created as a result of condensation of the gas flowing through the nozzle to produce unexpectedly high quantities of X-radiation. Moreover, it is stated that the soft x-ray emission has a much longer duration than that produced by direct emission processes. This suggests that the observed emission is a purely thermal process in which the electrons reach a sufficiently high temperature to strip the atoms down to the ionization state which can then emit the observed X-ray spectrum from recombination after cooling.
By contrast, it is one object of the present invention to provide an apparatus and method for generating intense prompt X-radiation.
Another object of our invention is to provide an apparatus and method for generating amplified X-radiation in which inner shell vacancies are produced by efficient non-thermal processes.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.